In some therapeutic treatments, in particular irradiating patients for the treatment of tumors, it is necessary to provide a plurality of catheters in the tissue. In the treatment of prostate tumors in men, for instance, it is common practice to pre-puncture the patient's skin with a sharp metal point before plastic needles are inserted in the prostate. In these needles, at a later time, radioactive sources are positioned in the catheters for irradiating a tumor located in the needles' vicinity without overloading the surrounding tissue. For the patient, a treatment with such a plastic needle is extremely traumatic, due to the blunt point being pressed through the usually harder tissue with force which is very painful. Due to the degree of pain normally experienced complete anesthesia is often required. Further, pressing a blunt needle with great force can result in wound fluid formation.